The first underwater hot spring to be discovered was probably just 40 years ago. In A.D. 1977, scientists discovered hot springs at a depth of over 8,000 feet, on the Galapagos Rift off the coast of Ecuador. Since then other hot springs have been found at several sites along the mid-oceanic ridges, many on the East Pacific Rise. They are mostly found at the active spreading centers along the mid-oceanic ridges, where magma, at temperatures over 1,000 C, presumably was being erupted to form new oceanic crust at places where tectonic plates are being pulled apart. The average temperature of deep-ocean water is only 2°C. This movement allows the molten magma to rise from deep inside the Earth, superheating the cold ocean water around it. The water coming directly from a hydrothermal vent can go as high as 350°C and is rich in dissolved chemicals.
These ocean springs are just a recent discovery but look more than three thousand years back what the Bible says in the book of Job chapter 38 verse 16:
Job 38:16: Have you entered into the springs of the sea, or walked in the recesses of the deep? ESV
Also, the existence of deep recesses in the ocean was not fully understood until recently, and now we know that there are Oceanic trenches which are topographic depressions of the seafloor, relatively narrow in width, but very long. Some are as deep as the Mariana Trench located in the western Pacific Ocean about 200 km east of the Mariana Islands and is the deepest natural trench in the world. The maximum known depth is 10,994 meters 36,070 ft.